The present invention relates generally to integrated circuits, and, more particularly, to a power supply switching circuit for an integrated circuit.
Electronic devices such as metering devices and data converters include internal circuitry powered by a main supply. Some of the internal circuitry, such as dynamic random access memory (DRAM) and system clock circuits, are required to operate in the absence of the main supply. One way of powering the internal circuitry is by using a battery when the main supply is cut-off such as in the event of power failure or when the electronic device enters a low power mode. However, batteries only hold charge for a limited time period.
Supply-switching circuits are commonly used to toggle between the main supply and the battery. A supply-switching circuit connects the internal circuitry to the main supply when the main supply is available and to the battery when the main supply is unavailable. The supply-switching circuit is required to accurately monitor the voltage levels of the main supply and the battery to determine the switching point.
FIG. 1A shows a schematic circuit diagram of a conventional supply-switching system 100 connected between a main supply 102 and a battery 104. The supply-switching system 100 includes a switching circuit 105 and a supply-selection circuit 106. The switching circuit 105 includes first and second transistors 108 and 110 and a first diode 112. The first and second transistors 108 and 110 are p-channel metal-oxide semiconductor (PMOS) transistors. The supply-selection circuit 106 includes an inverter 114 and a logic circuit 116. The logic circuit 116 is connected to the main supply 102 and the battery 104 for receiving the main supply and battery voltages, respectively. The logic circuit 116 includes a comparator (not shown) that compares the main supply and battery voltages and outputs a first control signal based on the comparison. The inverter 114 is connected to the logic circuit 116 for receiving the first control signal and generating a second control signal. The logic circuit 116 is powered by the main supply voltage and the inverter 114 is powered by the battery. A source terminal of the first transistor 108 is connected to the main supply 102 for receiving the main supply voltage. A gate terminal of the first transistor 108 is connected to the logic circuit 116 for receiving the first control signal. When the main supply voltage is available (i.e., when the main supply voltage is within a predetermined range), a voltage at a drain terminal of the first transistor 108 equals the main supply voltage. When the main supply voltage is unavailable (i.e., when the main supply voltage is not within the predetermined range or switched off), the voltage at the drain terminal of the first transistor 108 equals the battery voltage. Thus, a body terminal of the first transistor 108 is connected to the drain terminal thereof so that a voltage at the body terminal of the first transistor 108 is always greater than or equal to the main supply voltage received at the source terminal thereof, thereby meeting a body-bias voltage requirement of the PMOS transistor. A source terminal of the second transistor 110 is connected to the battery 104 for receiving the battery voltage. A gate terminal of the second transistor 110 is connected to the inverter 114 for receiving the second control signal. A body terminal of the second transistor 110 cannot be connected to the drain terminal thereof as either of the source or drain terminals of the second transistor 110 may be at a higher voltage at any point of time. Therefore, the first diode 112 is included having a p-terminal connected to the drain terminal of the second transistor 110 and an n-terminal connected to the drain terminal of the first transistor 108, which also forms an output terminal of the switching circuit 105. The first diode 112 ensures that the source terminal of the second transistor 110 is always at a higher voltage than the drain terminal thereof and reduces leakage of the battery 104. Thus, the body terminal of the second transistor 110 is connected to the source terminal thereof, thereby meeting the body-bias voltage requirement of the PMOS transistor.
When the main supply voltage is greater than the battery voltage, the first control signal sets the gate terminal of the first transistor 108 low, thereby switching on the first transistor 108. The second control signal sets the gate terminal of the second transistor 110 high, thereby switching off the second transistor 110. When the battery voltage is higher than the main supply voltage, the first control signal sets the gate terminal of the first transistor 108 high, thereby switching off the first transistor 108, and the second control signal sets the gate terminal of the second transistor 110 low, thereby switching on the second transistor 110. The first transistor 108 conducts the main supply voltage and the second transistor 110 conducts the battery voltage to the output terminal of the switching circuit 105, when the respective transistors are switched on.
However, the supply-selection circuit 106 selects the battery 104 when the battery voltage is greater than the main supply voltage, even when the main supply voltage is within the predetermined range. Additionally, the first diode 112 introduces a diode drop (e.g., 0.7 V) causing a reduction in the battery voltage obtained at the output terminal of the switching circuit 105. Use of additional electronic components, such as the comparator, further increases the cost of production and overall chip area.
FIG. 1B shows an alternate implementation of the conventional supply-selection system 100. The supply-selection system 100 of FIG. 1B includes the switching circuit 105, a control logic circuit 117, and a supply-selection circuit 118 that selects and provides the higher of the main supply and battery voltages to the switching circuit 105. The supply-selection circuit 118 includes second and third diodes 120 and 122. The second diode 120 has a p-terminal connected to the battery 104 and an n-terminal connected to an n-terminal of the third diode 122 to form an output terminal of the supply-selection circuit 118. A p-terminal of the third diode 122 is connected to the main supply 102. The output terminal of the supply-selection circuit 118 is connected to the control logic circuit 117 and body terminals of the first and second transistors 108 and 110 for outputting a selected voltage. The control logic circuit 117 is connected to the gate terminals of the first and second transistors 108 and 110 for generating first and second control signals, respectively, based on the selected voltage. When the battery voltage is greater than the main supply voltage, the second diode 120 conducts the battery voltage as the selected voltage to the output terminal of the supply-selection circuit 118. Upon receiving the battery voltage as the selected voltage, the control logic circuit 117 generates the first control signal at logic high state and the second control signal at logic low state. As a result, the first transistor 108 is switched off because its gate terminal receives the first control signal, which is at logic high state. The second transistor 110 is switched on because its gate terminal receives the second control signal, which is at logic low state. When the main supply voltage is greater than the battery voltage, the third diode 122 conducts the main supply voltage as the selected voltage to the output terminal of the supply-selection circuit 118. On receiving the main supply voltage as the selected voltage, the control logic circuit 117 generates the first control signal, which is at logic low state, and the second control signal, which is at logic high state. As a result, the gate terminal of the first transistor 108 is low, thereby switching on the first transistor 108, and the gate terminal of the second transistor 110 is high, thereby switching off the second transistor 110. The body terminals of the first and second transistors 108 and 110 always receive a higher voltage, since the output terminal of the supply-selection circuit 118 outputs the higher of the main supply and battery voltages as the selected voltage, and hence, the need for the first diode 112 in the switching circuit 105 is eliminated. However, the second and third diodes 120 and 122 introduce a diode drop (e.g. 0.7 V) causing a reduction in the selected voltage, and hence, the first and second control signals do not completely switch off either of the first and second transistors 108 and 110.
FIG. 1C shows yet another implementation of the conventional supply-selection system 100. The supply-selection system 100 of FIG. 1C includes the switching circuit 105, the control logic circuit 117, and a supply-selection circuit 124 that selects and provides a higher of the main supply and battery voltages to the switching circuit 105. The supply-selection circuit 124 includes third and fourth transistors 126 and 128. The third transistor 126 has a source terminal connected to the battery 104 for receiving the battery voltage, a body terminal connected to its drain terminal, and a gate terminal connected to the main supply 102 for receiving the main supply voltage. The fourth transistor 128 has a source terminal connected to the main supply 102 for receiving the main supply voltage, a body terminal connected to its drain terminal, and a gate terminal connected to the battery 104 for receiving the battery voltage. The drain terminal of the fourth transistor 128 is connected to the drain terminal of the third transistor 126 to form an output terminal of the supply-selection circuit 124. The output terminal of the supply-selection circuit 124 is connected to the control logic circuit 117 and body terminals of the first and second transistors 108 and 110 for outputting the selected voltage. The control logic circuit 117 is connected to the gate terminals of the first and second transistors 108 and 110 for generating the first and second control signals, respectively, based on the selected voltage.
When the battery voltage is greater than the main supply voltage, the gate terminal of the third transistor 126 is low. The third transistor 126 is switched on and conducts the battery voltage as the selected voltage to the output terminal of the supply-switching circuit 124. Upon receiving the battery voltage as the selected voltage, the control logic circuit 117 generates the first control signal, which is at logic high state and the second control signal, which is at logic low state. As a result, the gate terminal of the first transistor 108 is pulled down, thereby switching off the first transistor 108 and the gate terminal of the second transistor 110 is pulled down, thereby switching on the second transistor 110. When the main supply voltage is greater than the battery voltage, the gate terminal of the fourth transistor 128 is at logic low state. The fourth transistor 128 is switched on and conducts the main supply voltage as the selected voltage to the output terminal of the supply-switching circuit 124. Upon receiving the main supply voltage as the selected voltage, the control logic circuit 117 generates the first control signal, which is at logic low state and the second control signal, which is at logic high state. As a result, the gate terminal of the first transistor 108 is pulled down to logic low state, thereby switching on the first transistor 108 and the gate terminal of the second transistor 110 is pulled down to logic high state, thereby switching off the second transistor 110. The output terminal of the supply-switching circuit 124 always outputs the higher of the main supply and battery voltages as the selected voltage to the body terminals of the first and second transistors 108 and 110, thereby meeting the body-bias voltage requirements of the first and second transistors 108 and 110. However, when the difference between the main supply and battery voltages is small, the gate terminals of the third and fourth transistors 126 and 128 are both at logic high states, thereby simultaneously switching off the third and fourth transistors 126 and 128. As a result, the logic states of the first and second control signals are indeterminate and hence, the first and second transistors 108 and 110 are set to an indeterminate state.
FIG. 1D shows yet another implementation of the conventional supply-selection system 100. The supply-selection system 100 of FIG. 1D includes the switching circuit 105, the control logic circuit 117, and a supply-selection circuit 130 that selects and provides the higher of the main supply and battery voltages to the switching circuit 105. The supply-selection circuit 130 includes fifth and sixth transistors 132 and 134, fourth and fifth diodes 136 and 138, and first and second fixed-current sources 140 and 142. The fifth transistor 132 has a source terminal connected to the battery 104 and a p-terminal of the fifth diode 138, a body terminal connected to its drain terminal, and a gate terminal connected to an n-terminal of the fourth diode 136. The sixth transistor 134 has a source terminal connected to the main supply 102 and a p-terminal of the fourth diode 136, a body terminal connected to its drain terminal, and a gate terminal connected to an n-terminal of the fifth diode 138. The drain terminal of the sixth transistor 134 is connected to the drain terminal of the fifth transistor 132 to form an output terminal of the supply-selection circuit 130. The n-terminal of the fourth diode 136 is connected to the first fixed-current source 140 and the n-terminal of the fifth diode 138 is connected to the second fixed-current sources 142 for tracking the main supply and battery voltages, respectively. The output terminal of the supply-selection circuit 130 is connected to the control logic circuit 117 and body terminals of the first and second transistors 108 and 110 for outputting the selected voltage. The control logic circuit 117 is connected to the gate terminals of the first and second transistors 108 and 110 for generating the first and second control signals, respectively, based on the selected voltage.
When the battery voltage is greater than the main supply voltage, the gate terminal of the fifth transistor 132 receives the main supply voltage by way of the fourth diode 136, which is less than the battery voltage received at the source terminal thereof. Hence, the gate terminal of the fifth transistor 132 is at logic low state. The gate terminal of the sixth transistor 134 receives the battery voltage by way of the fifth diode 138, which is greater than the main supply voltage received at the source terminal thereof. Hence, the gate terminal of the sixth transistor 134 is at logic high state. As a result, the sixth transistor 134 is switched off and the fifth transistor 132 is switched on. The fifth transistor 132 conducts the battery voltage to the output terminal of the supply-selection circuit 130. On receiving the battery voltage as the selected voltage, the control logic circuit 117 generates the first control signal at logic high state and the second control signal at logic low state. As a result, the gate terminal of the first transistor 108 is at logic high state, thereby switching off the first transistor 108 and the gate terminal of the second transistor 110 is at logic low state, thereby switching on the second transistor 110. When the main supply voltage is greater than the battery voltage, the gate terminal of the sixth transistor 134 is at logic low state and the gate terminal of the fifth transistor 132 is at logic high state, thereby switching off the fifth transistor 132 and switching on the sixth transistor 134. The sixth transistor 134 conducts the main supply voltage to the output terminal. On receiving the main supply voltage as the selected voltage, the control logic circuit 117 generates the first control signal at logic low state and the second control signal at logic high state. As a result, the gate terminal of the first transistor 108 is at logic low state, thereby switching on the first transistor 108 and the gate terminal of the second transistor 110 is at logic high state, thereby switching off the second transistor 110. The output terminal of the supply-switching circuit 130 always outputs the higher of the main supply and battery voltages as the selected voltage to the body terminals of the first and second transistors 108 and 110, thereby meeting the body-bias voltage requirements of the first and second transistors 108 and 110. The fourth and fifth diodes 136 and 138 introduce diode drops (e.g. 0.7 V) at the gate terminals of the fifth and sixth transistors 132 and 134, respectively. Hence, the gate terminals of the fifth and sixth transistors 132 and 134 cannot be at logic high state at the same time. Therefore, the fifth and sixth transistors 132 and 134 are not simultaneously switched off even when the difference between the main supply and battery voltages is small. However, the first fixed-current source 140 receives a constant current from the battery 104, causing considerable leakage of the battery 104 and decreasing the battery-life.
It would be advantageous to have a supply-selection system for selecting between a main supply and a battery that selects the main supply when the main supply is within a predetermined range, and selects the battery only when the main supply is out of the predetermined range.